1. Field of the Invention
The present invention relates to apparatuses and methods for processing images for decoding coded video information and outputting the information as analog baseband signals or digital baseband signals.
2. Description of the Related Art
Image pickup apparatuses such as digital video cameras mainly adopt Moving Picture Experts Group (MPEG) 2 as a coding method. Moreover, a large number of image recording apparatuses and image reproducing apparatuses support the MPEG-2 coding.
In the MPEG-2 coding, in general, quantization errors and the like cause more coding noise in reproduced images as the compression rate of the images is increased, resulting in degradation in image quality. Typical coding noise includes mosquito noise and block noise as high-frequency noise generated in high-frequency regions. In order to remove such coding noise and improve the quality of reproduced images, decoded images are filtered using low-pass filters (coding-noise reduction filter) in some technologies (for example, see Japanese Patent Laid-Open No. 2002-232889).
On the other hand, when image pickup apparatuses, Digital Versatile Disc (DVD) players, or the like are used as image output apparatuses and connected to image display apparatuses such that images are reproduced and displayed, the image output apparatuses and the image display apparatuses are connected to each other via S-terminal connectors, D-terminal connectors, or the like. In general, the image output apparatuses convert digital video signals into analog video signals, and output analog baseband signals to the image display apparatuses.
Recently, connection interfaces typified by High-Definition Multimedia Interface (HDMI) capable of transmitting digital baseband signals in addition to the analog baseband signals have been in widespread use.
FIG. 6 is a block diagram schematically illustrating a configuration of an image processing apparatus that decodes coded video information and outputs analog baseband signals or digital baseband signals. The structure and operations of an image processing apparatus 110 will now be described with reference to FIG. 6.
Video signals (a so-called MPEG stream) compressed and coded using the MPEG-2 coding method are input to an MPEG decoding unit 114 via an input terminal 112. The MPEG decoding unit 114 reconstructs image signals in each frame or each field by decoding the compressed and coded video signals from the input terminal 112.
A filter-strength determination section 118 in a noise reduction unit 116 determines the strength of a noise filter 120. The filter strength indicates, for example, the cutoff frequency of the noise filter 120. The noise filter 120 is, for example, a spatial low-pass filter (LPF) used for digital processing, and removes or reduces coding noise in the video signals output from the MPEG decoding unit 114 using the fixed filter strength specified by the filter-strength determination section 118.
An output-format determination unit 124 operates a switch 122 in response to, for example, a selection instruction issued by a user.
The switch 122 outputs the video signals passed through the noise filter 120 to a digital encoder 126 or a National Television System Committee (NTSC) encoder 130.
When digital baseband signals are output, the digital encoder 126 converts the digital image data output from the noise filter 120 into digital baseband signals, and outputs the signals to an output terminal 128. The digital baseband signals are output from the output terminal 128 to an external device.
On the other hand, when analog baseband signals are output, the NTSC encoder 130 converts the digital image data output from the noise filter 120 into digital baseband signals in the NTSC format. Subsequently, a D/A converter 132 converts the digital signals output from the NTSC encoder 130 into analog signals, and supplies the signals to an anti-aliasing filter (AAF) 134. The AAF 134 performs anti-aliasing on the analog signals output from the D/A converter 132 so as to remove high-frequency components. The analog baseband signals processed by the AAF 134 are output from an output terminal 136 to an external device.
In this manner, high-frequency components need to be removed from the analog baseband signals such that aliasing is prevented. The application of the anti-aliasing filter causes differences in the degree of the coding noise generated in the digital baseband signals and the analog baseband signals. When the AAF 134 has an ideal frequency response whose gain at a frequency lower than or equal to the cutoff frequency is 1.0 and whose gain at a frequency higher than the cutoff frequency is zero with respect to a sampling frequency fs, the band width of the output digital baseband signals corresponds to that of the output analog baseband signals. However, it is impossible to realize an anti-aliasing filter having such an ideal frequency response.
FIG. 7 illustrates a spectrum of digital image signals output from the MPEG decoding unit 114. As shown in FIG. 7, the digital image signals output from the MPEG decoding unit 114 include components repeatedly appearing at each sampling frequency fs. FIG. 8 illustrates an ideal frequency response of an anti-aliasing filter applied to analog signals into which digital signals including such cyclic components are converted. As shown in FIG. 8, the gain at frequencies from a direct-current frequency to the cutoff frequency fs/2 is 1.0, and the gain at frequencies higher than or equal to the cutoff frequency fs/2 is zero. That is, the frequency response completely removes components whose frequencies are higher than or equal to the cutoff frequency fs/2.
In order to achieve such an ideal cutoff characteristic, the number of taps needs to be infinite. However, that is impractical. As shown in FIG. 9, an anti-aliasing filter with a finite number of taps has a frequency response whose gain is gently reduced from a frequency immediately lower than the cutoff frequency fs/2. When the AAF 134 has the frequency response shown in FIG. 9, the passband of the analog baseband signals output from the AAF 134 corresponds to a portion indicated by a hatched area shown in FIG. 10. That is, the gain at a frequency immediately lower than the cutoff frequency fs/2 is smaller than the original value, and furthermore, the gain is reduced as the frequency approaches the cutoff frequency fs/2.
When the noise filter 120 for reducing the coding noise is disposed upstream of the AAF 134 as shown in FIG. 6, the spectral characteristic of the analog baseband signals output from the AAF 134 corresponds to that obtained by overlapping the frequency response of the noise filter 120 with that of the AAF 134. FIG. 11 illustrates the frequency response 150 of the noise filter 120, the frequency response 152 of the AAF 134, and a frequency response 154 obtained by synthesizing the frequency responses 150 and 152. The abscissa represents the frequency, and the ordinate represents the gain.
The noise filter 120 and the AAF 134 are applied to the analog baseband signals. That is, the frequency response 154 obtained by synthesizing the frequency response 150 of the noise filter 120 and the frequency response 152 of the AAF 134 is applied to the analog baseband signals. A hatched area shown in FIG. 12 (inner zone of the frequency response 154 shown in FIG. 11) corresponds to the passband of the output analog baseband signals.
On the other hand, a hatched area shown in FIG. 13 (inner zone of the frequency response 150 shown in FIG. 11) corresponds to the passband of the output digital baseband signals since only the noise filter 120 is applied to the digital baseband signals. That is, the passband of the digital baseband signals is wider than that of the analog baseband signals. Herein, the frequency response of the transfer function of the digital encoder 126 is regarded as being flat.
As described above, the strength of the noise filter for reducing the coding noise is set to the same value when either analog or digital images are output in the known method for removing noise. Therefore, the output bands of the analog baseband signals and the digital baseband signals differ from each other due to the effects of the anti-aliasing filter. This leads to differences in the degree of noise in reproduced images.